“If what we find holds in general, it will become very difficult to distinguish traits that are adaptations from the traits that are not adaptations,” Wagner said.
Even prior to this study, the two concepts — adaptation versus exaptation (drawn from nonadaptive traits or traits adapted for another purpose) — were difficult to separate. Gould and Vrba acknowledged that one can lead to the other, and that any complex feature contains both.
Others, however, say it’s impossible to distinguish adaptation from exaptation, rendering Gould and Vrba’s definition of exaptation redundant. “Nothing was ever designed for what it is currently used for,” said Greger Larson, an evolutionary biologist at Durham University. He and his colleagues identify a decline in the use of exaptation relative to adaptation in evolutionary biology literature and blame the trend on the lack of a clear distinction; they propose redefining the term.
The murkiness of past selection pressures makes it challenging to say that any trait was ever truly adaptive. Birds’ and bats’ wings could be called exaptations of arms; however, the structural changes that followed cannot be called adaptations because “you are talking about a historical incident; it’s not something you can test,” said Mark Norell, a vertebrate paleontologist at the American Museum of Natural History, who studied with Vrba.
However, some counter that exaptation and adaptation are indeed distinct, meaningful phenomena, although the distinction can be subtle. “Indeed, (virtually) everything is a modification of some previous form,” Thornton wrote in an email. “But that’s not the point.” The defining factor, he and others said, is the action of natural selection.
Thornton offered two examples: If new mutations enable an enzyme to detoxify a pesticide present in the environment, the detoxification activity is an adaptation; that is, it emerged as a result of natural selection. On the other hand, if a hormone once in charge of regulating one process is co-opted to regulate a second process, that is an exaptation because the hormone did not evolve by natural selection to regulate the second process.
The strength of Barve and Wagner’s theoretical approach was that they could definitely demonstrate the potential for exaptation outside of any historical context. By randomly assembling metabolic networks, they were able to sidestep the evolutionary baggage that would accompany real microbes. But to truly assess the role of exaptation in evolution, they will need to validate their results in living organisms. That’s what they hope to do next, though exactly how remains to be seen. “We are still trying to figure that out,” said Wagner. “It is a really hard problem.”
Reprinted with permission from Quanta Magazine, an editorially independent division of SimonsFoundation.org whose mission is to enhance public understanding of science by covering research developments and trends in mathematics and the physical and life sciences.